Physiological illusions can arise in perfectly healthy mentally people under the influence of affective states - stress, fatigue, or due to a decrease in the degree of attention. Illusions of a physical and physiological nature are considered episodic, short-lived, and instantly critical.
At the same time, a person's ability to come to a critical assessment is considered the most important and basic criterion for distinguishing between simple illusions and disorders of a pathological mental state, which psychiatrists are studying.
The orienting reflexes of a person are the result of exposure to various external specific stimuli, manifested as muscle contractions related to a specific organ, which brings it to a position that most closely matches external influences.
A person's perception of reality, when he believes that he "sees" or "hears", is a report of consciousness due to stimulation of certain receptors of specific perceiving elements (retina of the eye, organ of Corti, etc.). At first glance, these processes should be in full compliance with each other (harmonious correspondence of objective manifestation and subjective perception), but this is not the case.
It is the discrepancies between objective real indicators and an erroneous or false perception of a subjective nature that create physiological illusions. The most striking example of a natural physiological illusion is a violation of the eye gauge - the inability to accurately determine "by eye" the real dimensions of distances, objects - discrepancies in the readings of perception consciousness and their objective parameters.
Also, a physiological illusion is called an erroneous perception of the visually reduced size of a large object if it is in a group of small elements, and vice versa. A well-known physiological effect is often demonstrated by psychologists and ophthalmologists - when pressing a finger from the side on the eyeball, all objects in a person's perception are bifurcated.
That is, all physiological illusions can be called special effects of exaggerated stimulation of the organs of vision and the brain (color, brightness, movement, position, size, tilt). Any physiological illusion can arise under the influence of a stimulus inadequate in quality or strength on any sensory organ.
Experts refer to a physiological illusion as a frequently occurring phenomenon - "the illusion of counter-rotation", which appears in a person when the semicircular canals of the vestibular apparatus are exposed to changes in the rotational speed in a particular plane (for example, in astronauts, pilots, in the case of a long ride on a carousel). The direction of perception of the illusion of counter-rotation is directly opposite to the actual process of rotation.
Any illusion of counter-rotation is accompanied by vegetative disorders, sensations (false, imaginary) of rotation of one's own body and objects, often a person feels unpleasant changes in consciousness, a feeling of fear and confusion appears, attention is scattered.
To suppress these sensations, it is necessary to show volitional effort, in order to avoid - long-term training of the vestibular-vegetative apparatus, reflexes (this, of course, applies to pilots and astronauts, so that they have the opportunity during the "blind" flight not to react to the illusion, but to navigate devices).
Physiological illusions, being more or less known to everyone, are easily corrected by mentally healthy people, and only in rare cases lead to false and imaginary perceptions. By themselves, such solitary physiological illusions do not cause a delusional state, are not sources of its manifestation or further development. And as an additional sign, they can only intensify delirium that has already developed as a result of mental illness, as a perverted external painful perception.
According to D. N. Uznadze, one of the forms of physiological illusion is the illusion of the attitude, the wrong perception of the mass, volume of objects, of any size. This illusion is created by repeatedly comparing several pairs of objects; to obtain the desired result, a series of experiments is carried out, creating the necessary prerequisites for the appearance of a physiological illusion of the installation, that is, the conditions of the installation itself.
So, for example, when each hand simultaneously lifts a pair of objects that differ in weight, and then another pair, with the same weight, the illusion of the installation arises that the object in the hand, in which there was a lighter object before, is heavier (contrasting physiological illusion of the installation).
D. Uznadze explains the mechanisms of the emergence of the illusion of attitude by the theory of persistent formation in a person of internal subconscious perceptions (attitudes), preparing him for the subsequent, already imaginary perception of further events.
Some physiological illusions of perception are characterized by a complex nature of manifestation, for example, a situation of weightlessness, when stimulating vestibular sensations is unusual and unfamiliar to a person's consciousness, violates the visual and acoustic assessment of the position of objects (such phenomena include physiological illusions of time, touch, temperature, color, etc.).
To this day, there is no general theory explaining the mechanisms of physiological illusion. Illusory effects, according to the theory of the German scientist G. Helmholtz, are the result of the functioning under unusual conditions of the same constant mechanisms of perception, which, under familiar conditions, provide a stable reality.
Scientists explain the manifestations of physiological illusions by the specifics of the coding processes in the consciousness of information, the structural features of the eye, light effects, the appearance of irradiation, contrasts.
A change in the characteristics of the perception of physiological illusions can be interpreted as a definition of the global characteristics and personal qualities of a person - his emotional state in critical situations of perception (at the moment of fatigue or activity), his character, and even determine the type of personality, pathological changes, the status of a person, his inherent self-esteem , a tendency to suggestion.
There are many examples of physiological illusions. There is an illusion of color perception, known to many psychologists, in which a dark figure on a light background will always seem smaller in size than a light figure on a dark background, although in reality they are the same. This effect was used by duelists in ancient times, they preferred to come to a duel dressed in black suits, rightly believing that it would be more difficult to get into a smaller visually figure with a pistol.
Scientists explain this effect by irradiation - an image formed on the retina, consisting of light and dark zones, is distorted under the influence of the fact that light from brightly lit areas conventionally "moves" into dark areas. Due to this phenomenon, a light object against a dark background is perceived as larger than it is in reality, since it “captures” a section of a dark background.
Scientists and specialists in various fields are aware of the most diverse and interesting physiological illusions. Weber's illusion is that a cold object is always perceived as heavier than a warm object, which in reality has the same weight.
The illusion of the physiological nature of sound contrast is expressed in the perception of sounds of the same strength against a background of quiet sounds louder than against a background of louder ones.
The Mueller-Schumann illusion is the erroneous perception of a lighter load after repeatedly lifting a heavy load, even lighter than it is in reality, and, conversely, if you lift a light load several times, then the heavier one, lifted after that, seems heavier.
The Charpentier illusion deceives our consciousness in a visual-physiological way - lifting two objects of the same weight and appearance, having at the same time a different volume, a person will perceive the smaller of them in size - heavier.
The Bezold-Brucke effect is an illusion that is expressed in changes in the shade of light in cases of a change in its intensity. Especially yellow-red and yellow-green tones are perceived as more yellow when the light intensity is increased, while bluish-red and bluish-green are perceived as bluer. In pure reds, yellows, greens, and blues, this effect does not occur.
The Purkinje effect was discovered by him in 1825, when he drew attention to the change in the brightness of blue and red road signs at different times of the day. In the daytime, the colors were equally bright, and at sunset, the blue paint seemed brighter than the red. And at dusk, the colors fade completely, and are perceived by a person in gray tones. Red appears to be black and blue appears to be white. This effect is caused by the transition of cone vision to rod vision in low light.
The illusion of the Moon is that the apparent (or apparent) size of this celestial body (or the Sun) is perceived by a person to be approximately 1.5 times larger when the body is located low above the horizon than when it is high at the zenith. This is an example of a true physiological illusion. You can prove that this is a "game" of perception by trying to close the Moon at arm's length with your thumb. It will cover equally the ascending lunar disk and the celestial body at its zenith.
Unusually interesting is the McGurk effect, which practically proves that the auditory and visual information that speech conveys to a person's consciousness not only interact in consciousness with each other, but also affect what a person hears. Scientists McGurk and McDonald conducted an amazing experiment by creating conditions in which the auditory signal impulses of the spoken syllable did not correspond to the corresponding movements of the lips.
To do this, the participants in the experiment were shown a video of a person who uttered the syllables "ga-ga" several times with his lips only, and the phonogram voiced other syllables - "ba-ba". Listening to the phonogram with their eyes closed, the participants learned the correct syllables. When they visually perceived only the movements of the lips of the speaking person, with the soundtrack turned off, they also perceived the pronounced sounds as "ga-ga".
But, with the simultaneous presentation of conflicting auditory and visual stimuli, they heard sounds that were not actually present in any case at all, and what turned out to be even more interesting for scientists is that most of the participants in the experiment did not realize the discrepancy between the auditory and visual perception stimulation. Many experts call the McGurk effect a classic example of a physiological illusion, since it involves two senses at once.